> explaining for loops with goto/jumps seems a bit abstract to me
I assume you mean "concrete" - goto/jumps is how it's done in the machine code; to get less abstract we'd have to start looking at how the processor is built.
From the vantage point of the Ruby programmer, the only concrete things are the Ruby statements used to model your desired behaviour.
When you're teaching goto/jumps you'd have to either teach another programming language (raw instructions) which would make it more concrete, but would also raise a lot more questions about how Ruby works, or teach goto in terms of Ruby code, which would make it fairly abstract :P
I assume you mean "concrete" - goto/jumps is how it's done in the machine code; to get less abstract we'd have to start looking at how the processor is built.